This invention relates to a prosthetic hip implant system which comprises a part-spherical cup adapted for location in an acetabulum and having a part-spherical inner bearing surface to receive a part-spherical ball head which can be attached to a prosthetic stem for location in a femur and in which the inner bearing surface of the cup extends around an angle of more than 180°. Cups of this type are known which comprise a single bearing element made, for example, of a synthetic plastic material such as ultra high molecular weight polyethylene or metal. The inner bearing surface can also be formed on an insert which again can be of a synthetic plastic material, a ceramic, or a metal and which is carried in an outer housing, the outer housing engages the acetabulum with which it is to be used and being held in place by, for example, cement or by mechanical means, for example nails or screws. The cup is used in connection with Total Hip Arthroplasty which includes implanting a femoral component in the femur which component normally includes a stem, a neck and ball head.
In other known constructions of the cup the inner bearing surface can be provided on an inner liner or insert made from a different material from an outer backing which engages the acetabulum. Dual mobility cups or bipolar cups generally comprise an inner bearing which receives the part-spherical ball head and which inner bearing itself is freely rotatably mounted in a part-spherical housing which has an outer surface for engaging an acetabulum and an inner bearing surface. The bearing outer surface engages the inner bearing surface of the outer housing which engages the acetabulum. This type of cup allows greater mobility of angular movement. Such cups are shown in U.S. Pat. No. 4,798,610 and U.S. Publication 2004/0143341.
In all these constructions, and, more especially, dual mobility cups, it is advantageous to have a means for retaining the ball head against the inner bearing surface. There are several ways of retaining the ball head inside the inner bearing surface. For example, U.S. Pat. No. 4,798,610 uses a ring seated on a conically tapered surface in the inner bearing. The second arrangement can consist of having two flats on the head and turning the head at 90° prior to inserting it into the cup and then again turning the head back through 90°. A disadvantage with this type of construction is that specific heads are required and there is the risk of wear due to the truncating of the head. A system of this type is shown in FR 2 785 525 and U.S. Patent Publication No. 20030171817.
Another way for retaining the ball in a part-spherical cup adapted for location in an acetabulum having a part-spherical inner bearing surface to receive the part-spherical ball head which can be attached to a stem for location in a femur is by using flat surfaces on the cup and ball. The part-spherical inner bearing surface of the cup extends around an angle of more than 180°, and a portion thereof adjacent an entry mouth is formed with a substantially flat face which is at a radius from the center of the part-spherical inner bearing surface. This radius is less than the radius of the remainder of the cup, and the ball head has a co-operating substantially flat face on its part-spherical surface on which is provided a structure to receive and retain the stem with which it is to be used, and which prior to attachment to the stem allows it to be located in the cup and rotated so that it is retained thereon. The dimensions and configuration of the part-spherical inner bearing surface and the part-spherical bearing surface on the ball head being arranged to cause a movement of translation of the head during rotation to displace the head so that there is a crescent shaped retention area on each opposed side.
Such a construction is shown in U.S. Pat. No. 7,520,902. An advantage of this construction is that standard sized ball heads can be used. The entry into the cup can be closely controlled by the dimensions of the flat on the inner bearing surface so that the operation of the ball head against the bearing surface is accurate.
In a preferred construction the transverse axis of the mouth of the cup which is substantially parallel with the flat face of the ball head when being inserted is offset from the transverse axis of the inner bearing surface of the cup. The offset can be less than 10 mm, for example up to 5 mm.
The invention can be applied to cups and balls of any suitable material, for example synthetic plastics material, metals or ceramics. An amount, for example, 1 mm of free subluxation can be incorporated if necessary.